1. Field of the Invention
This invention relates to an improved cryosurgical apparatus and method and, in particular to apparatus and a method for improving the freezing and defrosting procedures utilized in cryosurgery.
2. Discussion of the Prior Art
Cryosurgery by definition is the use of extreme cold to perform various surgical procedures, most of which require the killing of living tissue. One early use of cooling by gas expansion described by Amoils was the removal of cataracts by freezing a small diameter probe to the cataract and then pulling to remove the cataract from the eye. Later Dr. I. S. Cooper used cryosurgery to destroy brain tissue as a treatment for Parkinson's disease. Then in the 1960's a device was designed to treat various gynecological disorders. This was a hand held unit which was equipped with interchangeable tips and operated by pressing a "trigger" in the handle which initiated coolant flow to cool the tip in contact with the tissue to be treated. These systems normally use carbon dioxide (CO.sub.2) or nitrous oxide (N.sub.2 O) for coolants because they are readily available and may be stored for indefinite periods.
The physical phenomena which produce the cooling are, in order of importance, evaporation and the Joule-Thompson Effect. Since the boiling points of nitrous oxide and carbon dioxide are -88.degree. C. and -78.degree. C. respectively at atmospheric pressure, they produce temperatures approaching these levels when they are expanded into a cryosurgical tip or probe through an orifice or expansion tube. The Joule-Thompson effect is the cooling generated when a high pressure gas is expanded through an orifice to a lower pressure. The Joule-Thompson coefficient relates change of temperature to change in pressure. EQU U=dT/dP
where
U is the Joule-Thompson coefficient PA1 dT is the change in temperature PA1 dP is the change in pressure.
The Joule-Thompson coefficient for nitrous oxide, for example, is 0.7165. If this is substituted into the equation to determine dT using full cylinder pressure of 50 atmospheres (750 psi) exhausted to the atmosphere, the resulting dT is: ##EQU1##
When this is applied at body temperature, which is nominally 37.degree. C., the body temperature is reduced to 1.degree. C. which is not low enough to generate tissue destruction. Tissue necrosis is generally accepted to occur at approximately -21.2.degree. C., the tissue eutectic temperature. Therefore, the basic contribution of the Joule-Thompson effect is to initially cool the tube which is feeding more gas to the expansion site, thus producing two-phase flow, i.e., liquid and gas, in the expansion tube. This mixture is sprayed onto the inner surfaces of the tip, evaporates, cools the tip, and is then exhausted. In route to the exhaust port the cold gas from the tip passes around the metal tube which is feeding the coolant to the tip. The incoming coolant in the metal tube is then super-cooled and liquefies, ultimately supplying 100% liquid to the tip. This is called regeneration and occurs in most cyosurgical instruments where feedline and exhaust are coaxial.
Thus, the Joule-Thompson effect produces enough cooling to initiate the freezing portion of a cryosurgical procedure and evaporation does the rest. This is also indicated by the fact that in well designed probes, tip temperatures often approach the boiling points of the coolants, i.e. about -80.degree. C. See "Anesthesiology", Vol. 38, No. 3, March 1973, pp. 280-282 for an additional discussion of the Joule-Thompson effect.
When the freezing portion of the procedure is completed, it is necessary to thaw the frozen area to remove the probe. To accomplish this conventionally the flow of the coolant is shut off downstream of the tip. With the gas flow stopped, the entire system is brought to pressure equilibrium (.perspectiveto. 750 psi) and since the tip is very cold, the gaseous coolant in the immediate area is liquified, thus warming the tip to just above the freezing point of water. A variation of this method is to completely reverse the flow of the coolant generating the same effect, i.e., transient exothermic liquid generation. Defrost temperatures generated by these means tend to be transient. That is, in the exhaust shut off method of defrost the temperature will usually rise to approximately 2.degree. C. and then begin to fall back below freezing. This becomes a significant problem if the gas supply pressure is higher than the nominal (750 psi at 68.degree. F.) in accordance with this invention as will be described in more detail hereinafter. As pressure increases defrost becomes increasingly ineffective, ultimately preventing ice ball release and removal from the operative site.
The mechanism of tissue destruction in cryosurgery is generally accepted to be rupture of the cell membrane resulting in cell death. It is well known that a cell can accommodate reduced temperatures by dehydrating into the intracellular interticies where freezing does no damage to the cell. This dehydration can be accomplished successfully by the cell only if the freezing rate is slow enough to permit adequate dehydration; higher freezing rates produces intercellular ice which expands on freezing and ruptures the cell membrane. Slow rates approximately 178 .degree. C. per minute are used to preserve biologic samples i.e. bovine sperm, etc. Faster rates generated by conventional cryosurgery systems are in the order of 5.degree. C./sec (300.degree. C./min) and usually cause unpredictable necrosis. This leads to the limited acceptance of cryosurgery as a viable means of treatment. This poor predictability then becomes a most significant problem of conventional cryosurgery.
This is worsened by the fact that as a conventional cryosurgical procedure progresses the pressure in the supply cylinder will decay. This is caused by evaporative cooling of the metal cylinder and its liquid contents as the liquid evaporates to supply coolant to the probe. Thus, the duration of the procedure is limited by gas pressure decay as is the possibility of doing repetitive procedures one after another since gas supplies must be changed or there must be a wait until the pressure recovers. Although the problem of decreasing gas pressure can be somewhat lessened by employing larger supply coolant cylinders, these are different to procure, fill and store, compared to the "E" sized cylinders which are universally available for all procedures.
Another shortcoming associated with the prior art is as follows. Conventionally depth of necrosis is judged by the thickness of ice generated around the tip during the procedure. Since ice generation requires only 0.degree. C. and the removal of the latent heat, this is a poor method because it does not adequately predict the depth of necrosis.